This invention relates to an apparatus and method for analyzing parameters of a fibrous substrate such as paper, and more particularly to evaluating the fiber orientation ratio, formation, and basis weight of paper.
In the papermaking art, the term "fiber orientation" is used to describe the general arrangement of fibers within a paper substrate. Paper fibers are not usually uniformly arranged in all directions in the plane of the paper. Instead, a proportion of fibers will be aligned in the "machine direction" ("MD") of the paper and others will be oriented perpendicular thereto, in the "cross direction" ("CD") of the paper. Other fibers will be oriented randomly between the machine direction and cross direction. Fibers in the machine direction will be those which are longitudinally aligned in the direction in which the paper traveled as a web in the paper manufacturing machinery. The cross direction is perpendicular to the direction of web travel through the paper making machinery.
Fiber orientation is a key factor in the strength and anisotropy of paper. Factors which influence fiber orientation include the design of the head box which holds the paper pulp, the paper pulp, the differential velocity between the pulp jet and the endless wire conveyor which transports the pulp in mat form, and the rate of dewatering used in the paper manufacturing process.
It has been demonstrated by various investigators that orientation of the fibrous material comprising a web has a distinct relationship to certain important physical properties of the web. Some of these properties are commonly known as tensile strength, Young's modulus, shear modulus, tension at rupture, normal strain, shear strain, bursting strength, etc.
For some products a random fiber orientation in large and small areas alike is desirable. In other products it is desirable to obtain a high ratio of fiber orientation in one direction as opposed to all other directions. Such a product would tear readily in a direction parallel to the highly oriented direction and provide maximum strength normal to the parallel direction.
Fiber orientation during web manufacture is controlled by adjusting the velocity of the pulp furnish jet relative to the velocity of the web forming wire. Orientation randomness is maximized by matching the two velocities exactly. Orientation in the machine direction is maximized by precisely controlling the wire and jet at slightly different velocities.
Several methods for determining the fiber orientation are known in the prior art. These include various kinds of tensile strength tests, in which the tensile strength in different directions of the paper are measured. Usable results are only obtained by these methods if a high correlation exists between tensile strength and orientation, which is not always the case. Another method for determining fiber orientation is the so called "staining" method. Stained fibers are added to the pulp and the fibers lying in different directions are individually counted. This technique is tedious, and is not suitable for use during the production of commercial products.
Various methods for the determination of fiber orientation based on the defraction of radiation are also known. For example, x-ray and neutron defraction methods have been studied, but their ultimate usefulness has not been determined. A difficulty present with such methods is that of interpreting the defraction patterns obtained. Ultrasonic techniques have also been employed to analyze fiber orientation, which utilize the production of waves in the paper.
In U.S. Pat. No. 3,807,868, a method is disclosed for determining the fiber orientation in paper by using reflected light. A polarized light beam, such as a laser beam, is directed at right angles against the plane of the paper. The intensity of the light reflected by the paper under a given angle is observed in two planes at right angles to each other. Two quantities are formed from the intensity of the reflected light. One results by conducting the reflected light to pass through a polarizer having a polarizing plane parallel to the plane of polarization of the light beam. The other quantity is obtained by conducting the reflected light to pass through a polarizer with its polarizing plane perpendicular to the plane of polarization of the light beam. The differences of the two quantities observed in both planes are calculated, and the ratio of these differences is used as a measure of the fiber orientation. The method disclosed in the '868 patent utilizes a stationary light source and detector arrangement.
Another parameter of interest in fibrous substrates such as paper is known as "formation". When a sheet of paper looks uniform on viewing it up to the light, its formation is called good, while an irregular, grainy or blotchy structure may be described as such or may be called "wild" formation. U.S. Pat. Nos. 3,435,240; 3,435,242; and 3,563,667 each disclose apparatus for measuring the formation of paper. In the '240 and '242 patents, two photomultipliers are directed toward respective large and small areas on a moving web of paper. The fields of view of the photomultipliers overlap such that the larger surrounds the smaller. The transmittance from a small spot of light on the paper is compared with that of a much larger area of light thereon, to provide a signal representing formation. In the '667 patent, a formation measurement is made by passing a light beam through the paper and letting it act on a light sensitive element on the other side thereof. A current is produced which is proportional to the light intensity and is composed of direct current and alternating current components. The alternating current component represents the non-uniformity in paper formation.
Another characteristic of paper which is of importance is its basis weight. The basis weight of paper, expressed in pounds, is the weight of a ream of 500 sheets of size 24" by 36". Because of inherent imperfections in the manufacturing process, the paper industry has for years been plagued by variations in basis weight across the sheet during manufacture. Various apparatus for measuring the basis weight of paper are disclosed in U.S. Pat. Nos. 3,207,901; 3,687,802; and 4,098,641. The '802 patent determines basis weight by using a gamma or beta ray gauge. The '901 and '641 patents both determine basis weight by passing light through a moving web of paper. The amount of light transmitted through the paper substrate is a function of the paper's basis weight.
It would be advantageous to provide a method and apparatus for determining various parameters of a fibrous substrate, such as fiber orientation, formation, and basis weight. Such apparatus and method should be highly accurate to provide meaningful results either on line in a manufacturing process or as part of a quality control procedure for a manufactured substrate. Equipment having this capability would provide immediate quantitative feedback to the web maker. Such feedback could ultimately provide a closed loop to insure the constant jet to wire velocity ratio so important to the manufacturer. The present invention relates to such a method and apparatus.